The present invention relates to print image enhancement techniques used in dot matrix printing machines, such as electrophotographic printing machines, and, more specifically, relates to modification of print signals to produce a printed image with visually softened edge contours.
Typically, non-impact printing machines are designed to create a printed image by printing a series of picture elements, pixels or dots, on a print medium, such as paper. In electrophotographic printing machines, laser printers for example, a desired image may be created by a light source which is caused to scan across the charged surface of photosensitive material in a succession of scan lines. Each scan line is divided into pixel areas and the light or laser beam is modulated such that some pixel areas are exposed to light and some are not, resulting in a predetermined series or pattern of overlapping pixels on each scan line. Wherever a pixel area is illuminated by the laser beam, the photosensitive material is caused to bear a charge pattern of pixels which images the subject that is being printed or reproduced. The printed copy is then obtained by developing the charged pattern and transferring the developed image to the print media.
The printed image produced by a dot matrix printer is a digitized or quantized image, sometimes referred to as a bit map image, of a desired analog image. The components of an analog image may be continuous in any orientation, while those of a bit map image must proceed in orthogonal, incremental steps. This constraint results in distortion in the bit map representation of an analog image. The bit map image typically consists of a large number of discrete pixels or dots organized in a predetermined (spatial) pattern.
The resolution of the bit map images produced by dot matrix printing machines is typically stated in the number of pixels or dots printed per inch. For example, a 300 dot-per-inch (dpi) printer has a higher resolution than a 240 dpi printer. A printer which produces 300 dpi in the horizontal row direction and 300 dpi in the vertical column direction has a 300 by 300 dpi resolution. At a resolution of 300 by 300 dpi, lines printed parallel or perpendicular to the scan direction print with very little visible distortion. However, diagonal lines produce jagged steps or staircase distortion which is quite visible to the human eye.
Distortion in bit map representations is a consequence of low resolution of the bit map or low sampling rates of the desired analog image. A typical approach to reducing this distortion has been to increase the resolution of the bit map image by increasing the number of dots in a fixed size image, i.e., reducing the dot size, which increases the spatial resolution. Increasing the resolution reduces the size of the step distortions as well as preserving much fine detail which is lost at lower resolution. However, increasing the resolution is expensive. The amount of data to be processed and stored is proportional to the number of pixels or cells in the bit map. For example, doubling the resolution of 300 by 300 dpi two-dimensional bit map results in a 600 by 600 dpi bit map which requires four times more memory and processing power. Further, a bit map image output device, such as a cathode ray tube (CRT) or printer, capable of displaying this higher resolution image must be used which may further increase the cost. While this solution has been used in many more sophisticated, high-end printers, it is not a practical solution for lower cost, low-end printers.
Another approach to reducing staircase-type distortion has been to modify the data signal which is used to actuate the display device, see, e.g., Bassetti et al., U.S. Pat. No. 4,460,909; Bassetti et al., U.S. Pat. No. 4,544,264; Watanabe et al., U.S. Pat. No. 4,544,922; Kawamura, U.S. Pat. No. 4,553,173; Bassetti et al., U.S Pat. No. 4,625,222; and Tung, U.S. Pat. No. 4,847,641; each of which is hereby specifically incorporated by reference for all that is disclosed therein. The present invention also utilizes the technique of data signal modification.